docs: def merge(x, y):
      """
      Merge two dictionaries and raise an error for inconsistencies.
  
      Parameters
      ----------
      x : dict
          dictionary x
      y : dict
          dictionary y
  
      Returns
      -------
      x : dict
          merged dictionary
  
      Raises
      ------
      ValueError
          if `x` and `y` are inconsistent
      """
      keys_x = set(x)
      keys_y = set(y)
  
      for key in keys_y - keys_x:
          x[key] = y[key]
  
      for key in keys_x & keys_y:
          value_x = x[key]
          value_y = y[key]
  
          if isinstance(value_x, dict) and isinstance(value_y, dict):
              x[key] = merge(value_x, value_y)
          else:
              if value_x != value_y:
                  raise ValueError
  
      return x